Hydrotreating processes are used by petroleum refiners to remove heteroatoms, such as sulfur and nitrogen, from hydrocarbonaceous streams such as naphtha, kerosene, diesel, gas oil, vacuum gas oil (VGO), and reduced crude. Hydrotreating severity is selected to balance desired product yield against the desired low levels of heteroatoms. Increasing regulatory pressure in the United States and abroad has resulted in a trend to increasingly severe and/or selective hydrotreating processes to form hydrocarbon products having very low levels of sulfur.
Hydrotreating is generally accomplished by contacting a hydrocarbonaceous feedstock in a hydrotreating reaction vessel, or zone, with a suitable hydrotreating catalyst under hydrotreating conditions of elevated temperature and pressure in the presence of a hydrogen-containing treat gas to yield a product having the desired level of sulfur. The operating conditions and the hydrotreating catalysts used will influence the quality of the resulting hydrotreated products.
It is conventional, i.e., known to those skilled in the art of hydrotreating and other forms of hydroprocessing, to separate and recycle at least a portion of the unreacted hydrogen in the hydrotreated product so that it can be combined with the fresh treat gas (also known as make-up gas) and the hydrocarbon feed. This separation is accomplished in, for example, a flash drum or separator vessel downstream of the hydrotreating reactor. It is also desirable to improve the purity (concentration) of hydrogen in the recycle stream. Thus, it has been the goal of the art to provide enhanced efficiencies of hydrogen utilization with little additional energy consumption and without undue deleterious effects on the maintenance or operation of the hydrotreating equipment. It has also been recognized that by increasing the efficient use of hydrogen, existing equipment could be employed to increase the throughput of the feedstock resulting in higher product yields. A further advantage to the more efficient utilization of hydrogen is the reduction in the amount of make-up hydrogen that must be provided by, for example, a hydrogen plant or cryo-unit.
The type of feedstock to be processed, product quality requirements, yield, and the amount of conversion for a specific catalyst cycle life determines the hydrogen partial pressure required for the operation of a hydrotreating unit. The unit's operating pressure and the treat gas purity determine the hydrogen partial pressure of the hydrotreating unit. Since there is limited control over the composition of the flashed gas from the downstream hydrotreater separator or flash drum, the hydrogen composition of the recycle flash gas limits the hydrogen partial pressure ultimately delivered to the hydrotreater reactor. When recycle is used, a relatively lower hydrogen partial pressure in the recycle gas stream effectively lowers the partial pressure of the hydrogen gas input component to the reactor and thereby adversely affects the operating performance with respect to product quantity and quality, catalyst cycle life, etc. To offset this lower performance, the operating pressure of the hydrotreating reactor has to be increased, which can be undesirable from an operational point of view. Conversely, by increasing the efficiency of hydrogen gas recovery and hydrogen concentration, the hydrogen partial pressure of the recycle gas stream is improved. This results in an overall improved performance of the hydrotreating process unit as measured by these parameters.
Some conventional methods have been proposed that attempt to improve the hydrogen utilization efficiency of the hydrotreating unit by increasing the concentration of the hydrogen in the recycle gas stream. Such processes typically result in significant additional equipment costs and/or require significant changes in operating conditions, such as temperature and pressure, which typically results in increased capital and operating costs.
One process that has been adopted to improve the hydrogen purity of the recycle stream in a hydroprocessing unit is conventional pressure swing adsorption. See, for example, U.S. Pat. No. 4,457,384 issued Jul. 3, 1984 to Lummus Crest, Inc. However, in order to incorporate the PSA unit, the pressure of the reactor effluent gas stream must be reduced from about 2,500 psig (175.8 kg/cm2) to about 350 psig (24.6 kg/cm2). Although the purity of the recycle hydrogen stream can be increased to about 99 mol %, the recycled gaseous stream must be subjected to compression to return it to 2,500 psig (175.8 kg/cm2) before introduction into the hydroprocessing feed stream. The net result is that the capital, operating and maintenance costs are substantially increased by the addition of a large compressor that is required when using a conventional PSA unit.
Another method is described in U.S. Pat. No. 4,362,613 to MacLean which uses membranes with pressure drops up to 150 atmospheres and which also incurs substantial capital investment and operating costs.
There is therefore a need for an improved process for enhancing the efficiency of hydrogen utilization by means that are compatible with existing hydrotreating units. It is desired that such a process would not adversely affect the hydrotreater throughput or the overall economies of the system, including capital expenditures and operating expenditures, the latter including maintenance and energy consumption.
In other words, although various hydrotreating processes are practiced commercially, there is still a need in the art for improved hydrotreating processes that can be practiced more efficiently and with higher reactor throughput.